Field of the Invention
This disclosure is generally related to extended Ethernet passive optical networks. More specifically, this disclosure is related to a method and a system for scheduling in cascaded PONs.
Related Art
In order to keep pace with increasing Internet traffic, network operators have widely deployed optical fibers and optical transmission equipment, substantially increasing the capacity of backbone networks. A corresponding increase in access network capacity, however, has not matched this increase in backbone network capacity. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks still presents a severe bottleneck in delivering high bandwidth to end users.
Among different competing technologies, Ethernet passive optical networks (EPONs) are one of the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics, offering the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. With the high bandwidth of optical fibers, EPONs can accommodate broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, because Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and consequently require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. The “first mile” is generally a logical point-to-multipoint network, where a central office serves a number of subscribers. For example, an EPON can adopt a tree topology, wherein one trunk fiber couples the central office to a passive optical splitter/combiner. Through a number of branch fibers, the passive optical splitter/combiner divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers (see FIG. 1).
Transmissions within an EPON are performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which can be an external network belonging to, for example, an Internet service provider (ISP) or a local exchange carrier. An ONU can reside either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are coupled to a one-by-n (1×n) passive optical coupler, where n is the number of ONUs, and the passive optical coupler is coupled to the OLT over an optical link. One may use a number of cascaded optical splitters/couplers to increase the number of ONUs. This configuration can significantly save on the number of fibers and amount of hardware.
Communications within an EPON include downstream traffic and upstream traffic. In the following description, “downstream” refers to the direction from an OLT to one or more ONUs, and “upstream” refers to the direction from an ONU to the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, data packets are broadcast by the OLT to all ONUs and are selectively extracted by their destination ONUs. Moreover, each ONU is assigned one or more logical link identifiers (LLIDs), and a data packet transmitted by the OLT typically specifies the LLID of the destination ONU. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler to the OLT.
Due to the limitations on optical power budget and fiber availability, in many cases, extended PONs with longer reaches and higher densities are needed.